Added documetation on function return types.

Author: ahalbert

doc/Language/functions.pod6

@@ -174,6 +174,19 @@ values.
     put b.perl;
     # OUTPUT«\("a", "b", "c")␤»
 
+=head2 Return Type Constraints
+
+Perl 6 has many ways to specify a function's return type:
+
+    sub foo(--> Int)      {}; say &foo.returns # (Int)
+    sub foo() returns Int {}; say &foo.returns # (Int)
+    sub foo() of Int      {}; say &foo.returns # (Int)
+    my Int sub foo()      {}; say &foo.returns # (Int)
+
+Attempting to return values of another type will cause a compilation error.
+
+    sub foo() returns Int { "a"; }; foo; # Type check fails
+
 =head2 X<Multi-dispatch|declarator,multi>
 
 Perl 6 allows you to write several routines with the same name, but different